Hydraulic fracturing of oil bearing formations has been practiced commercially for many years. Conventional hydraulic fracturing techniques involve pumping a fluid at a sufficiently high pressure and volumetric rate through a well hole lined with a steel pipe and into a hydrocarbon bearing zone to cause cracks to form and propagate within the surrounding geological formation. Although both oil-based and water-based fracturing fluids are available, water-based fracturing fluids are generally more economical, and they offer greater control over a broader range of physical properties than oil-based fluids. Water-based fracturing fluids are now generally preferred by the hydrocarbon retrieval industry. The following discussion and the present invention is directed to water-based fracturing fluids.
Fracturing fluids generally contain several components. Among the most important components are a proppant, which is a granular solid material, and a gellant, which controls rheological properties of the fracturing fluid. Proppants are typically chosen from highly rounded natural silica sand and from ceramic materials such as alumina. Alumina is preferred whenever compressive forces are expected to be high. Numerous other additives found in fracturing fluids include pH buffers, surfactants, clay stabilizers, biocides, and fluid-loss additives. Many of these specific chemicals used in the fracturing process are described in Chemicals in Petroleum Exploration and Production II, North American Report and Forecasts to 1993, Colin A, Houston and Associates, Inc., Mamaroneck, N.Y. (1984).
A primary purpose of fracturing fluids is to distribute the proppant in cracks formed and propagated during fracturing, causing them to remain open after the pressure is released. Uniform distribution of proppant in cracks tends to greatly increase the permeability of a geological formation, especially of a very tight formation, and enable a greater recovery and higher flow rate of hydrocarbons contained within the formation.
Hydraulic fracturing has become a relatively predictable practice. The orientation and lengths of cracks can, under certain circumstances, be substantially predetermined and controlled. The Petroleum Engineering Handbook, H. B. Bradley, ed., Society of Petroleum Engineers, Richardson, Tex., Chap. 55 (1987) presents a useful background discussion of hydraulic fracturing.
While the term "gellant" is in common use in the hydrocarbon recovery industry in connection with fracturing fluids, the term should not be taken literally to mean that fracturing fluid gellants form conventional nonflowing gels. Fracturing fluid gellants may be more appropriately classified as viscosifiers and rheology control agents. A primary purpose of the gellant is to maintain the proppant in suspension during fluid preparation, pumping, and distribution into the well hole and cracks generated within a hydrocarbon bearing formation. Gellants therefore should function under diverse shear conditions. For example, several hundred thousand liters of fracturing fluid may be injected into a well at pumping rates as high as 7950 L/min. Ideally, the viscosity of the fluid should be low during fluid mixing and pumping to minimize the energy required during these operations. The viscosity should be high enough, however, so that the proppant does not fall out of suspension and is delivered to its desired location. High temperatures in hydrocarbon bearing zones further complicate the rheological properties and requirements of fracturing fluids.
The hydrocarbon recovery industry generally employs fracturing fluids that exhibit reduced viscosity as shear conditions increase. The relatively higher viscosity exhibited at lower shear conditions helps to maintain the proppant in suspension, while lower viscosity exhibited under higher shear conditions improves fracturing fluid flow rate and distribution.
Fluid behavior characteristics of a fracturing fluid can be described by the following equation: EQU .tau.=K.gamma..sup.n,
where .tau. is the shear stress, K is the consistency index, .gamma. is the shear rate, and n is the fluid behavior index. When the value of n is 1, the fluid is Newtonian; when the value of n is less than 1, the fluid is thixotropic; and when the value of n is greater than 1, the fluid is dilatant. Thixotropic fluids having values of n around 0.4 to 0.8 are typically preferred for fracturing fluids. Newtonian fluids do not generally carry proppant.
Gellants are usually based on water soluble derivatives of common polysaccharide materials such as guar gum, cellulose, or xanthan. Hydroxypropyl guar (HPG) and carboxymethylhydroxypropyl guar (CMHPG) are two common guar derivatives that are frequently employed as gellants. Cellulosic materials commonly employed as gellants include hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, and hydroxypropylmethyl cellulose.
Well conditions, particularly well temperatures, have significant bearing on the choice of gellant. Hydroxypropyl guar is most useful at lower temperatures, and carboxymethylhydroxethyl cellulose is frequently employed under at higher temperature conditions. Hydroxyethyl cellulose and xanthan have intermediate temperature tolerances.
Recovery from deeper wells that typically involves higher operating temperatures presents challenges and requires greater control over the rheological properties of fracturing fluids. In general, increasing the gellant concentration in the fracturing fluid results in increased viscosity. Practical, economical, and operational considerations, however, limit the amount of gellant that can be introduced to a fracturing fluid to increase its viscosity. Additionally, excessive gellant polymer loading may result in poor mixing efficiency and substantial frictional resistance. Crosslinking agents have been employed to circumvent some of these gellant limitations.
Crosslinking agents are now conventionally used in fracturing fluids to modify their rheological properties. Some crosslinking agents operate on a timedelayed basis to increase the fluid viscosity at the bottom of a wall, after the fluid has passed through the great bulk of the well casing. Crosslinkers that are currently used include polyvalent metal salts that form chelates, such as borates, aluminates, titanates, chromates, and zirconates. Different crosslinkers exhibit different pH and temperature limitations that affect their usefulness under certain fracturing conditions.
After the fracturing fluid has been distributed in the well and the associated fracture formations, the non-proppant fracturing fluid residue is removed from the formation, while the proppant remains distributed in the fractures. Oxidizing agents and enzymes that attack the gellant are commonly used to hasten removal of the fracturing fluid residue. Temperature conditions may be determinative of the gel-breaking mechanism to be employed. For example, enzymes are useful at temperatures of up to about 50.degree. C. Oxidants such as sodium or ammonia perfulate and calcium or sodium hypochlorite are useful at temperatures of up to about 80.degree. C. In situ well temperatures above about 135.degree. C. may be sufficient to cause gel breakdown as a result of thermal degradation without the aid of a catalyst.
Although substantial research efforts have been devoted to developing hydraulic fracturing fluids that exhibit the desired stability and rheological properties, the results have not been entirely satisfactory. The present invention is therefore directed to fracturing fluids that provide improved rheological properties and control under various fracturing conditions.